It is well known to use semiconductor switches such as silicon controlled rectifiers (SCR's), bidirectionally conductive latching switches (Triacs), or transistors connected in switching configurations to control the average AC energization of electrical loads. In the most elementary systems, the current control device (SCR, Triac, etc.) is switched to a conducting state every cycle or half cycle of energization current. The device is then returned to a nonconducting state at another point in the cycle or half cycle (typically the end of each half cycle). Variation in average load current is achieved by varying the point in the cycle at which the device is switched to a conducting (or nonconducting) state. Examples of systems operating in such a manner are disclosed in U.S. Pat. Nos. 3,065,388 and 3,159,737 issued to B.H. Pinckaers and R. A. Dora on Nov. 20, 1962 and Dec. 1, 1964 respectively.
A disadvantage of the above-described scheme is that there is typically a sharp discontinuity in the flow of current at least once each cycle resulting from the requirement that switching must be accomplished at least once each cycle at an instant when the current flow would not ordinarily be zero. Such discontinuities cause electrical noise which interferes with the operation of sensitive electrical equipment. Filter apparatus for minimizing or removing this noise adds to the complexity and cost of the system.
The noted disadvantage can be minimized, at the expense of some control current complexity, by providing for alternate conduction and nonconduction of current in bursts of varying integral numbers of half cycles. Thus, all switching occurs at zero phase angle when current flow is ordinarily zero. Current control circuits operating in this manner are also well known as evidenced by U.S. Pat. Nos. 3,319,152; 3,443,124 and 3,633,094, the first two of which issued to B. H. Pinckaers on May 9, 1967 and May 6, 1969, and the last of which issued to R. E. Clements on Apr. 15, 1970.
Either type of control circuit may be employed with or without specific provision for feedback signals indicative of the state of operation of the load. In closed loop or servoed systems, feedback signals are typically utilized to produce error signals which control the load current. Although a form of short circuit protection may be provided through use of feedback signals, such protection is generally not effective where conduction of the current control device is terminated only at the end of a half cycle of energization current.
Where the load comprises a single branch device the parameter of principal concern is position or level of operation. Load current control is limited to varying the magnitude of AC current supplied to the branch. Where the load comprises plural branches, independent control of current magnitude through each branch may be required, as is the case in the system of U.S. Pat. No. 3,204,113 issued to A. S. Snygg on Aug. 31, 1965.
In elementary systems of the latter type (e.g., a reversible drill motor control circuit), a manually operable switch may be provided between the current control device and the load whereby the current can be selectively supplied to one of a plurality of load input terminals. Such an arrangement is not suitable in a fully automatic servoed system such as may be used for positioning an aircraft control surface. Positioning motors frequently used in such an application comprise two phase, reversible, variable speed motors of a type having a feedback signal generator which produces an alternating voltage in which amplitude is indicative of motor speed and signal phase relative to an AC reference signal is indicative of direction of rotation. Accordingly, the current control circuitry for use in such a servomechanism system must be capable of utilizing the described feedback signal.
The applicant has provided a unique variable burst length, switching current control circuit for use in an aircraft servomechanism system. In line with the foregoing discussion, the control circuit is capable of utilizing a variable amplitude, two phase AC feedback signal, and is capable of providing effective instantaneous short circuit protection.